1. Field of the Invention
The present invention relates to a dry etching method of a III-V Group compound semiconductor, more particularly, to a dry etching method of such a compound semiconductor using a plasma etching apparatus.
2. Description of the Prior Art
A III-V Group compound semiconductor such as gallium arsenide, indium phosphide, or gallium aluminum arsenide is used as a base material of a semiconductor laser, a light-emitting diode or the like. When a substrate of such a compound semiconductor is used, a semiconductor device can be manufactured such as a field effect transistor or an integrated device thereof having a better performance than that of a semiconductor device having a silicon substrate. Furthermore, by using such compound semiconductors, an "optoelectric circuit" has been developed with a new concept in which an electrical integrated circuit and a light emitting/receiving element circuit are integrally formed.
In order to improve the productivity and the reproducibility in the manufacture of semiconductor devices including such compound semiconductor devices, a dry etching method has recently been used in a manufacturing process thereof. Particularly, a reactive ion etching method using a parallel flat plate electrode type device has been suggested as a dry etching process. In this method, a plasma-generating gas containing halogen is introduced to a vacuum pressure of 10.sup.-2 to 10.sup.-1 Torr in a vacuum chamber having an electrode (cathode) to which high-frequency power is applied and a ground electrode (anode) opposite thereto. A material to be etched is placed on the cathode. When the plasma-generating gas is introduced, the high-frequency power is applied to the cathode, thereby generating a glow discharge. Then, the cathode is negatively self-biased by a mobility difference between electrons and ions, and a dark space is formed thereon. A cathode breakdown voltage Vdc is also generated by this self bias. Reactive ions generated by the glow discharge from the plasma-generating gas are accelerated by the cathode breakdown voltage, thereby bombarding the material to be etched. Atoms sputtered by this bombardment react with the reactive ions to generate highly volatile molecules, and the molecules are discharged from the vacuum chamber. Thus, the etching is performed.
In this etching method, the material can be anisotropically etched. Under optimal conditions, a pattern width of about 0.5 to 1 .mu.m can be obtained. Therefore, a high-density and high-performance semiconductor device can be obtained with this method in comparison to a wet etching method in which an undercut easily occurs and it is difficult to obtain micropatterning. However, although the dry etching method is effective in manufacturing a semiconductor device as described above, it has the problems to be described below when this method is used to etch a compound semiconductor unlike a single semiconductor material such as silicon, or metallic material such as aluminum. So, the dry etching method has not been used practically to etch compound semiconductors.
In a compound semiconductor, since this material consists of a plurality of different types of atoms, conditions for simultaneously etching those plurality of different types of atoms much be selected. Further, etching mechanism of the compound semiconductors is very complicated and is not known to a satisfactory extent.
In a manufacturing process of a semiconductor laser, in order to form an end face of a resonator or a mesa structure, an etching depth of about 5 .mu.m is required. In comparison to a prior art technique which needs an etching depth of 1 .mu.m or less, an etching rate and an etching selectivity with respect to a mask must be several times those of the prior art. Therefore, the prior art technique and the conditions thereof cannot be adopted.
CCl.sub.2 F.sub.2 or Cl.sub.2 gas is known as a reactive ion etching gas for GaAs. However, the present inventors confirmed by experiment that a GaAlAs film could not be etched by this gas. On the other hand, when a protection layer which does not contain Al was continuously formed on the GaAlAs crystal surface, the protection layer and the GaAlAs layer could be etched using Cl.sub.2 gas. However, such a protection layer adds another requirement for device design, and has an etching rate too low under anisotropic etching conditions. Therefore, such a protection layer cannot be used in a device manufacturing process.
The present inventors experimentally confirmed that when GaAlAs was etched by using CCl.sub.4 gas, an etching rate was too low and reproducibility was very poor.
The present inventors also experimentally confirmed that when a GaAlAs layer was etched using a gas mixture of Cl.sub.2 and CCl.sub.4, it was etched at an etching rate which was suitable for a device manufacturing process. Despite this, the present inventors found that proper etching could not be achieved by using the above-mentioned gas mixture for the following reasons.
(a) Since a proper etching rate can be obtained within a limited etching condition range, the proper etching rate cannot always be achieved due to variations in etching devices or different environmental factors. PA1 (b) This method has poor reproducibility. Since both the CCl.sub.4 and Cl.sub.2 gases cannot effectively scavenge H.sub.2 O, O.sub.2 and the like, Al reacts with O.sub.2 included in an etching atmosphere and H.sub.2 O is adsorbed in AlCl.sub.3 as a reaction product, resulting in poor evacuation. Therefore, an etching rate and etching characteristics thereof are easily changed by differences in the evacuation condition of H.sub.2 O, O.sub.2 and the like, reaction products, the evacuation efficiency of residues and the temperature of the electrodes. PA1 (c) An etching selectivity ratio of the compound semiconductor with respect to a mask cannot be set at a high value; it is 5 to 10 at most. This results from sputter of a mask since the ion bombardment effect of the gas is too large. PA1 (d) A polymer is formed on a surface of the etching material which is etched. This polymer partially serves as a mask with respect to the surface of the material, thereby blocking etching and complicating removal of this polymer after the etching.